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Additives for use in automotive
The market demand has led to a growing interest in plastics designed for special applications, incorporating additives and loads. Automotive is one of the markets that most requests to plastics. This is the subject of a paper presented by Loredana merchant, of Iberian LVL, in the seventh days of plastic automotive organized by the Centre Spanish of plastics. The day was by title thermoplastic compounds for the automotive sector.
In the sector of the automotive sector, the thermoplastic compounds more used are the compounds autolubricados and the materials conductivos. The self-lubricating compounds of employ when the thermoplastic material is exposed to the friction and to the wear. The additive employees endow to the thermoplastic of a special dynamic behaviour. The characteristics of these materials measure employing the following parameters: the coefficient of friction (static and dynamic), the factor of wear and the limit PV or thermal sensors unsteadiness. The main additive employees in these compounds are the PTFE (politetrafluoretileno), silicone, fibre of carbon, fibre of glass, graphite and bisulfuro of molybdenum.
The PTFE dispersed evenly on the thermoplastic resin creates a surface layer with a low coefficient of friction and a high resistance to pressure. The best performance is obtained with rates of 10 to 20 percent, as higher percentages increase wear and tear. However, the use of this additive may lead to the effect slip/stick, consisting of an unstable behavior during boot.
The silicone tends to migrate to the surface of the thermoplastic parts injected, creating a lubricant protector. The Silicon reduces the coefficient of friction, get a factor of low wear and a higher PV limit.
A cost-effective solution is to merge the properties of PTFE and silicone, which reduces slip/stick and provides effective lubrication at high speeds.
The incorporation of a thermoplastic compound carbon fibers improving the mechanical performance of the piece at high temperatures and increases the PV limit of the material, without increasing wear.
It also increases the thermal conductivity and the injected piece is therefore able to better dissipate the heat generated by friction. It also improves the electrical conductivity.
Used traditionally as a lubricant for metal parts, although it is also used in thermoplastics, particularly in applications of pieces that are submerged in water. The graphite is suitable for contact with drinking water.
The effect of graphite is similar to the of the carbon fiber, with the exception that does not improve the mechanical properties as it does the carbon fibre and Yes improving substantially the anti-static properties. The coefficient of friction of a resin loaded with graphite is between that of a polymer without load and on a compound, PTFE and silicone.
A cost-effective solution is to merge the properties of PTFE and silicone, which reduces slip/stick and provides effective lubrication at high speeds
In the injected parts, the outer layer remains in the amorphous phase, thereby reducing the compound tribological properties. The main effect of the molybdenum bisulfide is to act as agent nucleante: forms a crystalline layer on the surface which improves the properties of the compound.
This type of load is used in semi-crystalline polymers.
The incorporation of the fiberglass improves the mechanical and thermal performance (such as carbon fiber) and raises the limit PV. Although it can also increase the wear of the piece. This negative effect can be balanced with additives lubricants.
Glass fiber increase the coefficient of friction.
The thermoplastic conductivos employ to reduce the risk of downloads electroestáticas in automotive sector. The additives employed to increase the conductivity in this sector are the fibre of carbon, the graphite, the fibres of stainless steel and the black of coal.
The graphite (natural or synthetic) employs because of his properties lubricantes in a proportion no more elevated of the 30 percent. Addition to amorphous materials, increases the dimensional stability.
The fibres of carbon, derived of the poliacriolinito, rayón or brea, employ to increase the rigidity of the material. They possess the advantage that they wear out less than the fibres of glass.
The fibres of stainless steel (of 8 - 12µm of diameter) present an optimum resistance and resilencia. The reduction of the electromagnetic interferences (EMI) produce by dissipation and absorption. The compound that incorporate this additive are easy to process.
Finally, employs the black of coal (carbon black), fibres of carbon with appearance of a fine dust (13 - 60 nm).
The best autolubricante
The parameters that are used to measure the benefits of a thermoplastic compound high are as follows:
- Coefficient of friction. It is the resistance to the movement of a body on a surface. -The force necessary to begin the movement - can be static, or dynamic - force necessary to keep the movement. In the case of the thermoplastic materials the dynamic coefficient is greater than the static.
- Limit PV. Specifies the maximum product of pressure and speed beyond which appears a thermal instability followed by a failure of the component.
- Factor of wear. It is a ratio that indicates the resistance to wear and tear on the material. The factor of wear is measured with the same instrument that is used to set the PV limit and is based on the loss of weight of the worn out piece.
On the other hand, the surface finish affects on wear surface, depending on the roughness (to more roughness, most wear) and used material (wear of aluminum is greater than that of steel)